As a lubricant for a bearing, lubricating oil or grease is generally and frequently used. As the base oil composing the main component of the lubricating oil or the grease, it is possible to list mineral oil and synthetic oil such as poly-α-olefin oil, ester oil, silicone oil, and fluorine oil. In recent years, from the standpoint of energy conservation, there are growing demands that the lubricating oil or the grease for use in a bearing or the like has a low friction and a low viscosity. With an increase of demands that a bearing for electric and electronic components of a car is compact and has a high performance, there are growing demands that the lubricant enclosed in the bearing has an improved durability at a high temperature.
As arts suggesting the possibility of the solution of the above-described demands, the fluid bearing unit (see patent document 1) using the lubricant containing the ionic liquid as the conductivity imparting agent and the grease composition (see patent document 2) composed of the base oil containing the ionic liquid and the thickener are known. These arts have been developed by utilizing the property of the ionic liquid which is a molten salt at a normal temperature has a low viscosity owing to the combination of various organic ions.
Description is made in the patent document 1 that because the lubricant contains the ionic liquid which is the conductivity imparting additive, static electricity is not accumulated at the rotational portion, and a stable and low torque loss can be achieved even in the condition of a high-speed rotation and the like. But it is unclear as to whether because the lubricant contains the ionic liquid, the lubricant has the effect of making the viscosity of the lubricant low and the effect of improving the durability thereof at a high temperature. In the patent document 2, although the viscosity of the lubricant is decreased by using the ionic liquid, it is anticipated that the ionic liquid progresses the corrosion of the bearing steel. The durability of the bearing at a high temperature cannot be sufficiently enhanced by the use of the lubricant.    Patent document 1: Japanese Patent Application Laid-Open No. 2004-183868    Patent document 2: Japanese Patent Application Laid-Open No. 2006-249368